<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        .box {
            height: 500px;
            border: 1px solid black;
            width: 50%;
            margin: 20px auto 0;
            display: flex;
            justify-content: space-between;
            align-items: flex-start;
        }

        .box div {
            width: 100px;
            height: 100px;
            background-color: red;
            border: 1px solid black;
        }


        .left-bottom{
            align-self: flex-end;
        }

        .right-top{

        }
    </style>
</head>
<body>

<div class="box">
    <div class="left-bottom"></div>
    <div class="right-top"></div>
</div>

</body>
</html>